Visit this Solr wiki page for more information: https://cwiki.apache.org/confluence/display/solr/UpdateXmlMessages

You can use XML requests to:

• Delete documents by id: deletes a document that match the specified id.
  
  
• Delete documents by query: deletes documents that match the specified query.
  
  

Here's the structure of the delete operation:

<delete>
    <id>
    <query>

Here's a sample XML message to delete an existing document by its id:

<delete>
    <id>1</id>
</delete>

Here's a sample XML message to delete existing documents by query:

<delete>
    <query>id:1</query>
</delete>

To delete all documents:

<delete>
    <query>*:*</query>
</delete>

Here's an example of an http request to delete a document by id:

$ curl http://localhost:8983/solr/COLLECTION-NAME/update?commit=true \
-H "Content-Type: text/xml" \
--data-binary '<delete><id>1</id></delete>'

Here's an example of an http request to delete documents by query:

$ curl http://localhost:8983/solr/COLLECTION-NAME/update?commit=true \
-H "Content-Type: text/xml" \
--data-binary '<delete><query>*:*</query></delete>'

Here's an example of an http request that uses the parameter stream.body:

http://localhost:8983/solr/COLLECTION-NAME/update?stream.body=<delete><query>*:*</query></delete>

In order for this request to work, you need to enable Streaming in "solrconfig.xml":

<requestParsers enableStreamBody="true" />

If you want to update the "solrconfig.xml" file directly in the Configset, make sure to reload all collections that use the updated Configset.
Visit this page for more details: http://lucene.apache.org/solr/guide/8_5/requestdispatcher-in-solrconfig.html